1. Field of the Invention
The present invention relates to a thyristor with a self-protection function for breakover turn-on failure.
2. Discussion of Background
The thyristor has four regions with different conductivity types alternately arranged, i.e. P, N, P and N regions. A gate electrode, for example, is provided in an intermediate region of one of the four regions. In the semiconductor device of this type, a current flows through a path between the anode and cathode electrodes of a main thyristor by a signal applied to the gate electrode. An amplifying gate is provided near the gate electrode so as to facilitate the turn-on of the main thyristor by the gate signal in a normal state. When the amplifying gate is turned on, the main thyristor is turned on by its load current. Thus, the auxiliary gate prevents the main thyristor from being destroyed when the gate electrode is being supplied with the gate signal. When no gate signal is supplied, if an overvoltage in excess of a breakover voltage of the thyristor is applied to the thyristor, the main thyristor is often turned on prior to the turn-on of the auxiliary thyristor. In such a case, the firing region of the thyristor, unlike the amplifying gate portion, is generally in the shape of a spot, and hence has a small region which cannot expand quickly. For this reason, the thyristor cannot withstand a rush current, which results in the destruction of the thyristor.
For solving the above disadvantage, a semiconductor device with an additional amplifying gate, as disclosed in Japanese Patent Publication (KOKOKU) No. 56-41180 has been proposed. In this device, the additional or second amplifying gate is triggered by a leak current frequently generated around the peripheral part of the semiconductor substrate. An amount of the leak current is susceptible to a surface condition of the peripheral part of the substrate. Therefore, the turning on of the second amplifying gate is unstable.
U.S. Pat. No. 4,165,517 discloses another semiconductor device for the thyristor, in which the minority carrier lifetime in the substrate under the gate electrode is selected to be larger than that in the other part of the thyristor in order to prevent a turn-on failure of the semiconductor device. However, since a large current always flows through the region near the substrate region under the gate electrode when the thyristor is turned-on, this region generates more heat than other portions. With a rise in temperature, the breakover voltage tends to rise. Therefore, the protection for the destruction of the thyristor is not sufficient.